The objective of the invention is notably to propose a dynamic and adaptive method for controlling the compression parameters for a compressed video stream having a number of spatial or temporal scalability levels which correspond to different resolutions. Such a method makes it possible to adapt the bit rate of the video stream to be transmitted to a number of users or to a single user via a transmission channel which experiences available bit rate variations which are significant over time. Another objective of the invention is to perform an optimized allocation of the bit rate between, on the one hand, the compression and, on the other hand, the protection. The criterion retained, in all cases, to produce the optimization according to the invention is the quality perceived by the user or users.
In the case of a multimedia communication over a link that has a constrained bit rate and is subject to propagation errors, the optimization relative to a nominal operating point of the system is broadly sub-optimal because it does not take into account the variations of the conditions of the transmission channel, nor does it take into account the content of the multimedia stream to be transmitted.
The methods and the devices according to the prior art do not make it possible to adapt rapidly to the variations of the transmission conditions notably in the case of a wireless transmission. The coding parameters are generally determined for a given operating point, typically corresponding to an authorized/available average bit rate for a rate of losses or of residual errors undergone by the transmitted stream. When the transmission conditions are better than the retained operating point, a portion of the available bit rate is pointlessly allocated to the protection of the transmitted stream, whereas it could either be saved, or be used to send at a higher useful bit rate. Conversely, when the transmission conditions are less good than the retained operating point, an excessive proportion of the bit rate is employed by the unprotected useful data, which leads to inevitable losses in reception, that is to say that a portion of the data cannot be protected because the bit rate allocation for the protection in practice proves inadequate.
Another known solution which targets the problem of the variation of the transmission conditions relates to the adaptive coding and modulation (ACM) methods. These methods are based on an adaptation of the channel modulation and coding schemes according to the transmission conditions. One of their drawbacks is that they do not make it possible to take into account and modify the content of the stream to be transmitted to adapt it to the constraints of the channel. The adaptation in this case mainly has the effect of modifying the rate of transmission of the data which can lead to delay and jitter problems notably when the transmission is done at constant bit rate.
Nor do the prior art solutions take into account the particular feature of the video stream to be transmitted which can be represented at a number of resolution levels. Such a compressed stream offering the temporal or spatial scalability can be exploited to adapt the source to the requirements of each user in the case of a transmission through a heterogeneous network.
In order to overcome the limitations of the prior art solutions, the invention proposes a method that makes it possible to determine, optimally, compression parameters including the source bit rate, the protection rate and the resolution level of a compressed video sequence transmitted through a heterogeneous network and to a number of users. The determination of the parameters is done, notably, according to the needs of the users and the constraints, known a priori, of the transmission channels. The proposed method is implemented, for example, through a device called associated controller, the object of which is to determine and supervise said compression parameters.
The method according to the invention also makes it possible, for a given user receiving a compressed stream offering a temporal or spatial scalability, to assist in the decision regarding the choice of the subset to be decoded as a function of the perceived final quality. Furthermore, the invention remains compatible notably with the prior art solutions of ACM type for which it acts as a complement to the protection provided by this type of solution. With an ACM method causing the bit rate of the transmission channel to be made variable, the method according to the invention adapts, in this case, the useful bit rate of the transmitted stream according to the variable bit rate of the channel.
The benefit of the invention is notably that it provides tools allowing for the objective comparison of the perceived quality for each resolution transmitted in the compressed stream and thus makes it possible to assist in the decision of the choices of compression parameters associated with each resolution.